The durations (that is, the widths) of pulses produced from mode-locked lasers can be as short as a few femtoseconds. The response times of the fastest electronic circuits are thousands of times longer than the duration of these pulses and therefore electronic techniques are not used to directly measure pulse durations. One of the shortest events available for measurement purposes is the pulse itself and this can therefore be the basis of optical autocorrelation techniques used for ultrashort pulse measurement.
In the most common autocorrelator arrangement, an input pulse (that is, a parent pulse), passes into a Michelson interferometer, which first splits the parent pulse into two child pulses, which are substantially identical in shape, amplitude, and phase, that is, are coherent. The two child pulses then travel along separate paths in the interferometer, one path being of variable length by use of a reflecting arm with a variable position. The two child pulses exit the interferometer overlapped spatially but with a relative temporal delay equivalent to the difference in path lengths travelled by each respective identical child pulse.
A two-wave mixing process, such as second-harmonic generation, is used to obtain a mixing signal between the two child pulses. By studying how the mixing signal varies in response to changes in path length, a correlation signal, containing information about the amplitude and phase of the parent pulse can be obtained from which its duration can be determined.